1. Field of the Invention
This invention relates to an air/fuel ratio estimator for a multicylinder internal combustion engine, more particularly to an air/fuel ratio estimator for a multicylinder internal combustion engine for estimating the air/fuel ratios at the individual cylinders from an output of a single air/fuel ratio sensor installed at a confluence point of an exhaust system, which can easily be realized on a microcomputer mounted on a vehicle.
2. Description of the Prior Art
It is a common practice to install a single air/fuel ratio sensor constituted as an oxygen concentration detector in the exhaust system of a multicylinder internal combustion engine, and feedback control the detected air/fuel ratio to a desired air/fuel ratio by regulating the amount of fuel supplied to the engine. A system of this type is taught by Japanese Laid-Open Patent Publication No. Sho 59(1984)-101,562, for example.
When a single air/fuel ratio sensor is thus installed at an confluence point (the exhaust manifold joint) of a multicylinder engine such as having four or six cylinders, the output of the sensor represents a mixture of the values at all cylinders. This makes it hard to obtain the actual air/fuel ratio at each cylinder, and thus makes it difficult to converge the actual air/fuel ratio to a desired air/fuel ratio properly. Although this can be solved by providing the sensor for the individual cylinders, the arrangement will necessarily be expensive and what is more, brings another problem on sensor's service life.
For that reason, the assignee proposed an air/fuel ratio estimator using an exhaust gas model (discrete state-variable model) which describes the behavior of exhaust gas in a multicylinder internal combustion engine provided with a single air/fuel ratio sensor at its confluence point of the exhaust system (Japanese Laid-Open Patent Publication Hei 5(1993)-180,044; also filed in the United States on Dec. 24, 1992 under the Ser. No. of 07/997,769, now abandoned). A state equation concerning state variables of the model indicating air/fuel ratios at the individual cylinders is defined, and an observer is designed to reconstruct an unmeasured state variable, such that the air/fuel ratios at the individual cylinders are estimated accurately.
When realizing such an estimator on a digital computer, it becomes necessary to define a range of changes and a least value for each variable, since the digital computer has a finite word length such as 4, 8, 16, or 32 bits. In general, the range of change is determined from a possible maximum value that the variable concerned can physically be, and the least value of the variable is determined by dividing the possible maximum value by the digital computer's word length. When there is a possibility that an input or a computation result of the variable could exceed the range of change thus determined, the range is enlarged such that no excess would occur, and the least value is newly calculated in the manner explained above.
Thus, the so-called observer estimates an unmeasured state variable such that an error between the state variables of a control system and of the observer converges to zero. In the course of estimation, however, the estimated value may temporarily be a value that could never be in a real world. On the other hand, the resolving power in the estimation must be the same as the actual variable. Therefore, when implementing it on a low-performance, on-board microcomputer with fewer-word-length, there occurs an inconsistent problem that the resolving power of the variable becomes coarse if its range of change is made large, while the range becomes smaller if the resolution power is made fine. More specifically, when the variable's least significant bit (LSB) is assigned a small value so as to enhance accuracy, a possible maximum value of the variable is therefore limited to a certain extent, and hence the range of changes of the variable is automatically restricted.